Hot water noise and running shower

It sounds like you may have a combi boiler or whole house pump ? When you open the other tap a pump kicks in which increases pressure in the hot water system. The shower valve may be scaled up or wearing & needs replacing or servicing, this may be allowing the slight flow of water on the shower. Alternatively have you recently had a new boiler fitted ? Some shower valves aren't designed to cope with the added pressure of a combi boiler, especially if it's an old shower valve. Hope that helps you narrow it down a little.